Wavefront aberrations are the result of differences in optical path length between an ideal wavefront that would result in a perfect focus through an optical system, e.g., an opthalmic lens and the eye, and the aberrated wavefront that is produced by the actual optical system. Wavefront aberrations are often categorized by their radial order, which indicates the dependence of the aberration on pupil size. The optical components of the human eye suffer from both “low-order” and “high-order” wavefront aberrations. The low-order aberrations of interest to the correction of vision are actually a subset of the low-order aberrations, referred to as the “second-order” wavefront aberrations. Second-order aberrations vary as a function of the square of the distance from the center of the pupil or aperture. These wavefront aberrations are typically corrected with an eyeglass prescription that includes a spherical power component, a cylindrical power component and an axis component that describes the orientation of the cylindrical power.
High-order aberrations (HOAs) represent wavefront aberrations that vary as a cubic or higher order function of the distance from the center of the pupil or aperture. The HOAs of the eye include aberrations such as, for example, coma, trefoil and spherical aberrations. Although these aberrations are often smaller in magnitude than the low-order aberrations, HOAs of the eye can also degrade vision quality and limit visual performance.
It is possible to improve vision quality by correcting the HOAs of the eye in addition to the low-order aberrations. The eye examination and lens design procedures for conventional eyeglass prescriptions (Rxs), however, typically only yields a correction that minimizes the low-order aberration of the eye. This is generally true for single-vision lenses, bifocal lenses, and progressive lenses.
Progressive lenses, which are also referred to as progressive addition lenses (PALs), progressive power lenses, graduated prescription lenses, and varifocal or multifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation. They are characterized by a gradient of increasing lens power, added to the wearer's correction for the other refractive errors. Typically, the gradient starts at the wearer's distance prescription, in the upper portion of the lens and reaches a maximum addition power, or the full reading addition, in the lower portion of the lens. The final addition power is between 0.75 to 3.00 diopters for most wearers. The addition value prescribed generally depends on the level of presbyopia of the patient and is often closely related to age and to a lesser extent, existing prescription.
Referring to FIG. 1, general-purpose PALs typically belong to a class of surfaces that include four specific structural features: a distance zone 110, a near zone 120, a progressive corridor 130 and a blending region 140. Distance zone 110 is a stabilized region in the upper portion of the lens that provides the specified distance prescription. Near zone 120 provides a stabilized region in the lower portion of the lens that provides a specified add power for reading. Progressive corridor 130 connects distance zone 110 with near zone 120 and provides intermediate or mid-range vision. Blending regions 140 are peripheral regions of the lens and contain surface astigmatism, which produce blur and distortion and offer only minimal visual utility.